Ceramic matrix composites (CMCs) generally include a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications, such as in gas turbines, are silicon-based composites, which include silicon carbide (SiC) as the matrix and/or reinforcement material.
Different processing methods have been employed in forming CMCs. For example, one approach includes chemical vapor infiltration (CVI). CVI is a process whereby a matrix material is infiltrated into a fibrous preform by the use of reactive gases at elevated temperature to form the fiber-reinforced composite. For example, conventional cloth based CMCs formed by CVI typically have a porosity between 10 percent and 20 percent, a fiber volume fraction between 35 percent and 40 percent, and an interlaminar tensile (ILT) strength between 1 ksi and 3 ksi, as measured by a standard 1 inch diameter button pull test. CVI composite matrices typically have no free silicon phase, and thus have good creep resistance and the potential to operate at temperatures above 2,570 degrees Fahrenheit.
Another approach includes melt infiltration (MI), which employs molten silicon to infiltrate into a fiber-containing preform. For example, conventional unidirectional tape-based CMCs formed by MI typically have a porosity of below 3 percent, a fiber volume fraction between 20 percent and 33 percent, and an interlaminar tensile (ILT) strength between 5 ksi and 9 ksi. The matrix of MI composites contains a free silicon phase (i.e. elemental silicon or silicon alloy) that limits use of the CMC to below that of the melting point of the silicon or silicon alloy, or about 2,550 degrees Fahrenheit to 2,570 degrees Fahrenheit. Moreover, the free silicon phase causes the MI SiC matrix to have relatively poor creep resistance.
Another approach employs a partial CVI process followed by an MI process, and is generally referred to as “slurry cast MI.” This approach usually yields an intermediate porosity between that of MI composites and CVI composites, generally of about 6 percent, a fiber volume fraction between 35 percent and 40 percent, an interlaminar tensile (ILT) strength between 2 ksi and 4 ksi, and also contains residual free silicon phase within the composite matrix.